Track-following control system for magnetic disk drive

ABSTRACT

A track-following control system for a magnetic disk drive according to the present invention includes a magnetic disk, a magnetic head, a position error signal generator, a controller, and a stepping motor. One or more track position signals indicating a position of a track are recorded in the magnetic disk in numbers of sectors. The magnetic head writes and reads out normal data in and from the magnetic disk, and reads out the track position signal from the magnetic disk in the same manner. The position error signal generator generates a position error signal indicating a position error between the magnetic head and the track based on a track position signal. The controller calculates intra-sector errors by calculating an average position error of each sector and a desired number of estimated position errors of the corresponding sector by interpolation. Then, the controller produces, in numbers of the secotrs, position error signals having a larger number than that of the sectors by using a position error signal supplied from the position error signal generator. The stepping motor 18performs positioning of the magnetic head in accordance with the position error signals having the larger number than that of the sectors.

BACKGROUND OF THE INVENTION

The present invention relates to a track-following control system for amagnetic disk drive and, more particularly, to a track-following controlsystem using a sector servo method for a high track density magneticdisk drive.

Recently, a means for increasing the density of tracks concentricallyformed on a magnetic disk, for instance a floppy disk, has been employedin order to increase the recording capacity of the floppy disk as arecording medium of a floppy disk drive (FDD).

Information is recorded in and read out from the floppy disk by amagnetic head which can freely move above the surface of the floppydisk. Generally, the magnetic head is moved or seeks from a track on orfrom which information is currently being recorded or read out (to bereferred to as a current track) to a track on or from which informationis to be recorded or read out (to be referred to as a target track)under the control of a head positioning system using a servo controltechnique, and the magnetic head is positioned on the target track.

In the magnetic head positioning system using servo control, when themagnetic head is to be moved from the current track to the target trackand is to be positioned on it, the magnetic head is roughly moved closeto the target track without using the servo control (to be referred toas coarse positioning), and is then finely controlled by the servocontrol to follow the target track.

As the track density is increased, it is difficult to perform correcttrack following with a conventional open loop head positioning method.Therefore, a track-following system of a sector servo method using aclosed loop feedback technique has been recently developed and used inpractice.

According to the sector servo method, the tracks are divided into aplurality of sectors. A track position signal indicating the trackposition or servo information is recorded at part of each sector, i.e.,the beginning of each sector. The servo information is read by themagnetic head. A position error signal representing a deviation betweenthe position of the magnetic head and the track position, i.e., aposition error, is obtained by using the servo information. The magnetichead is driven by a servo mechanism to minimize the position error inaccordance with the position error signal, and follows the track. Morespecifically, in the track-following control system using the sectorservo method, the magnetic head is held at a position determined on thebasis of a previous servo information until a next adjacent servoinformation is obtained.

The number of bytes of data per sector must be variable in accordancewith the operating system (OS) of the information processing system fromthe viewpoint of information processing. Therefore, when the number ofbytes of data per sector is increased, the number of sectors isdecreased. As a result, the volume of servo information is decreasedand, accordingly, the number of position error signals obtained asdescribed above is also decreased. In order to increase the datarecording capacity of the floppy disk, a ratio of the data recordingarea with respect to the entire floppy disk area must be increased. Thenumber of position error signals is limited from this viewpoint as well.

When the number of position error signals is decreased in thetrack-following control system using the sector servo method, thefollowing problems arise because of the nature of the floppy disk.

The floppy disk has two modes of eccentricity, i.e., track distortion.The first mode eccentricity is caused by floppy disk exchange. Thesecond mode eccentricity is caused by the environmental changes, such asa change in temperature and humidity. Of these two eccentricities, thesecond mode eccentricity is particularly important in design of atrack-following control system.

FIG. 1 is a view for explaining a track distortion formed on a floppydisk by the environmental changes.

Referring to FIG. 1, a solid line 2a denotes a normal circular trackwithout deformation; a long-and-short-dashed line 2b and a broken line2c, represent deformed tracks, respectively.

Sectors 1--1 to 1--8 are formed to divide the circular track 2a into 8equal portions. Servo information SI1 to servo information SI8 arerecorded at the beginning of the sectors 1--1 to 1--8, respectively.

Since the floppy disk is made of a polymer material, it greatly expandsor contracts by a change in temperature and humidity. The floppy diskhas so-called anisotropy wherein the coefficient of expansion differs inthe vertical and horizontal directions. Therefore, as shown in FIG. 1,the circular track 2a in the normal condition is deformed to elliptictracks 2b and 2c when the floppy disk expands and contracts,respectively.

FIG. 2 is a developed view of FIG. 1. In FIG. 2, the angle of rotationof the floppy disk rotating in the direction of the arrow in FIG. 1 ismeasured with reference to the x-axis and plotted as the axis ofabscissa, and the distance between the intersection of the x-axis andthe track and a disk center C is plotted along the axis of ordinate toindicate the position of the track. The scale marks 1 to 8 on the axisof abscissa indicate servo information.

As shown in FIG. 2, the tracks 2b and 2c obtained when the floppy diskexpands and contracts, respectively, form waveforms of 2 periods perrevolution of the floppy disk.

FIG. 3 is a view for explaining the problems occurring in thetrack-following operation for the deformed track 2b when the floppy diskexpands.

A straight line 3 indicates the position or track of the magnetic headon the floppy disk immediately before start of the track-following modewhen the magnetic head has been coarsely positioned on the track 2b bycoarse positioning as described above. The long-and-two-short-dashedlines 4 and 4' drawn on two sides of the straight line 3 at equaldistances from it define a position error detectable range R which ispredetermined in the track positioning system. A position errorregarding a track falling outside the range R defined by the straightlines 4 and 4' cannot be detected.

In FIG. 3, since the servo information SI2, the servo information SI4,the servo information SI6, and the servo information SI8 of theeven-numbered sectors on the track 2b fall within the magnetic headposition error detectable range R, they can be detected. However, sincethe servo information SI1, the servo information SI3, the servoinformation SI5, and the servo information SI7 of the odd-numberedsectors on the track 2b fall outside the position error detectable rangeR, they cannot be detected. In other words, despite that the magnetichead is positioned at an average position of a target track, it cannotfollow the target track by the servo control since the track has adistortion due to the anisotropy in expansion coefficient of the floppydisk. As a result, data information cannot be regenerated.

This applies to contraction of the floppy disk because of the followingreasons. When an FDD is manufactured, its material and structural designare appropriately selected such that a change in position of themagnetic head caused by a change in temperature and humidity compensatesfor a change in position of the track caused by expansion or contractionof the floppy disk.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a track-followingcontrol system wherein the above drawbacks of the conventional techniqueare eliminated and good track-following characteristics can beconstantly obtained even when the number of bytes of data per sector ischanged.

It is another object of the present invention to provide atrack-following control system using a sector servo method, which isfree from the above drawbacks even when the number of sectors isdecreased.

The track-following control system for a magnetic disk drive accordingto the present invention comprises: a magnetic disk in which not lessthan one track position signal indicating a position of a track isrecorded in units of sectors; a magnetic head for writing and readingout data in and from the magnetic disk and reading out the trackposition signal from the magnetic disk; means for generating a positionerror signal indicating a position error between the magnetic head andthe track based on the track position signal; means for producing, inunits of the sectors, position error signals having a larger number thanthat of the sectors by using the position error signal supplied from theposition error signal generating means; and means for positioning themagnetic head in accordance with the position error signals having thelarger number than that of the sectors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2, and 3 are views for explaining the problems which are causedby track distortion in the conventional track following control system;

FIG. 4 is a block diagram showing an overall arrangement of atrack-following control system for an FDD according to an embodiment ofthe present invention;

FIG. 5 shows an arrangement of the recording areas of a floppy diskshown in FIG. 4;

FIG. 6 shows the recording format of part of FIG. 5 in detail; and

FIG. 7 shows the recording format of a floppy disk of a track-followingcontrol system according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will be described indetail with reference to the accompanying drawings.

FIG. 4 is a block diagram showing an overall arrangement of theembodiment of the present invention; FIG. 5 shows the arrangement of therecording areas of a floppy disk; and FIG. 6 shows a pattern of theservo information recorded on the floppy disk.

Referring to FIG. 4, a hatched portion indicates a section of part of afloppy disk 10 along the tracks. When the floppy disk 10 is rotated, thehatched section moves in the direction of arrow D with respect to amagnetic head 11.

The magnetic head 11 is connected to a stepping motor 18 through amechanical means 11A, and is driven by the stepping motor 18 in thedirection to cross the tracks of the floppy disk 10. The magnetic head11 reads information recorded on a track of the floppy disk 10 andsupplies it to a regeneration circuit 13 through an amplifier 12. Theregeneration circuit 13 regenerates the readout information and outputsit to a gate circuit 14. The gate circuit 14 classifies the informationsupplied from the regeneration circuit 13 into servo information andinformation including data but excluding servo information. The servoinformation is supplied to a servo pattern detector 15 through a line101. The information other than the servo information is supplied to adata processor (not shown) through a line 102.

The servo pattern detector 15 generates a timing signal based on theservo information supplied from the gate circuit 14, and outputs it to aposition error signal generator 16 through a line 104. The positionerror signal generator 16 generates a position error signal based on thetiming signal and outputs it through a line 105 to a controller 17including a microcomputer 17A and a memory 17B. The controller 17processes the position error signal in a manner to be described later,and outputs a control signal to the stepping motor 18 through a line106. The stepping motor 18 drives the magnetic head 11 in accordancewith the control signal supplied from the controller 17.

The gate circuit 14 supplies a control signal to the position errorsignal generator 16 through a line 103.

FIG. 5 shows an example of the floppy disk 10 having 16 sectors. Only 4tracks 21--1 to 21-4 are illustrated for the sake of convenience. Eachsector consists of a servo information area 20 for recording servoinformation or a servo pattern, and an area 22 for recording informationincluding data other than the servo information.

FIG. 6 is an enlarged view of a sector shown in FIG. 5. A servo patternconsisting of two types of servo information A and B is formed in theservo information area 20. Each of the information A and the informationB occupies adjacent halves of adjacent tracks so as to bridge them. Theinformation A and the information B are alternately recorded at theidentical positions of the tracks 21--1 to 21--4 in the longitudinaldirection thereof in the form of frequencies which are prime relative toa data signal frequency and distinguishable from each other.

The operation of the embodiment shown in FIGS. 4, 5, and 6 will bedescribed.

First, the magnetic head 11 is set at a position corresponding to theoutermost track 21--1, i.e., a position shown in FIG. 5 without settingthe track-following mode, i.e., before start of the seek mode. Thefloppy disk is rotated and informations on the track 21--1 are read bythe magnetic head 11.

The reason why the magnetic head 11 is positioned on the outermost track21--1 is that the eccentricity of the floppy disk is maximum there.

A signal read out by the magnetic head 11 is amplified by the amplifier12, regenerated by the regeneration circuit 13, and classified into dataand servo information signals by the gate circuit 14. The servo patterndetector 15 detects the relative positional relationship between theservo pattern and the magnetic head 11 in accordance with the inputservo information signal, and generates a timing signal. The positionerror signal generator 16 obtains a position error signal based on thetiming signal, and outputs it to the controller 17. The controller 17stores the input position error signals of M (M is an integer)revolutions of the floppy disk 10 in its memory 17B. Subsequently, usingthe stored position error signals of M revolutions, the controller 17calculates the average position error of each sector. This is done inorder to average the rotational variation of a spindle motor forrotating the floppy disk 10 and to remove the noise component.

Using the average position error of each sector, the controller 17calculates a desired number of estimated position errors of thecorresponding sector by interpolation. As a result, position errorsignals having a number larger than the number of sectors can beobtained and stored in the memory 17B.

A method to calculate an estimated position error by interpolation willbe described.

Assume that the position error signals of the respective sectors are{(SΔt): S=1, 2, . . . , N}where N is the number of sectors and Δt is thetime interval between the sectors.

Assume that Si=SΔt where i=1, . . . , N. An estimated position errorsignal X(S') at S' between S₁ and S_(N) can be approximated by aLagrange's interpolation polynominal L(S') as follows: ##EQU1## for,##EQU2## where π' is a differential of π. The Lagrange's interpolationpolynominal is successively calculated in accordance with the Aitkenmethod.

The interpolated value L(S') obtained in the above manner in stored inthe memory 17B as an intra-sector position error signal X(S') togetherwith a real or an actual position error signal X(S_(i)).

In the track-following mode, a quotient of a real position errorobtained based on the track position signal actually read out at thetarget track by the magnetic head, and a real position error obtainedfrom the outermost track and stored in the memory 17B, is calculated.Subsequently, the position error signal (estimated intra-sector positionerror) calculated for the outermost track and stored in the memory 17Bis multiplied by the above quotient. The obtained product is defined asthe estimated intra-sector position error of the target track and usedfor the track-following control together with the real position error.

The above calculation can be sufficiently performed in a real-timemanner according to the operation speed of the controller 17 comprisinga 1-chip microcomputer 17A.

Another embodiment having a similar arrangement as that of the aboveembodiment shown in FIG. 4 and a different control algorithm executed bythe controller 17 will be described.

In this embodiment, real position error signals of M revolutions of thefloppy disk 10 are stored in the controller 17 before start of thetrack-following mode. Subsequently, using the stored real positionerrors of M revolutions, an average real position error of each sectoris calculated in order to remove the noise component, and is defined as{X(SΔt): S=1, . . . , N}where N is the number of sectors and Δt is thetime interval between the sectors, i.e., the sampling period.

An average value X of the position error of one revolution of the floppydisk 10 is calculated. X can be calculated by ##EQU3## The deviationX(S) of the average value X(SΔt) of each sector from the average value Xis calculated by the following formula:

    X(S)=X(SΔt)-X

where S=1, 2, . . . , N

Assume an autoregressive model wherein X(S) is defined as a function ofprevious X(S-1) to X(S-M):

    X(S)=a.sub.1 ·X(S-1)+a.sub.2 ·X(S-2)+. . . +a.sub.M ·X(S-M)

An appropriate assessment function is selected, and the number of termsof the autoregressive model for minimizing the assessment function and acorresponding coefficient a_(i) are obtained. According to thesimulation for each track, the number of terms is not substantiallychanged by the amplitude and phase of the two modes of eccentricity ofthe floppy disk and values of the coefficients are not substantiallychanged. In time, the same equation can be used for all the tracks.

For example, when one track is divided into 30 sectors and therevolutions per minute of the floppy disk is 300 rpm, the samplingfrequency is 150 Hz. In this case, the bandwidth of the servo is about1/10 of the sampling frequency and can be about 15 Hz. The first andsecond modes of eccentricity are 5 Hz and 10 Hz, respectively. Thenumber of terms of the autoregressive model based on these modes ofeccentricity is 2, and its coefficients are:

    a.sub.1 =1,8858

    a.sub.2 =0.9878

An FPE (Final Prediction Error) is used as the assessment function tocalculate these a₁ and a₂.

When the number of sectors per track is decreased to increase the datarecording density of the floppy disk, the number of position errorsX(SΔt) is small and track-following accuracy is degraded by the trackdistortion. In order to prevent this, an estimated position error in arequired angle of rotation of each sector is introduced in the equationof the above autoregressive model in addition to the actual positionerror X(SΔt). Simulation is performed using this equation, and thenumber of terms and the coefficient of each term are calculated andstored in the memory 17B.

As described above, during the track-following operation, actualprevious position errors and estimated intra-sector position errors,i.e., position errors having a larger number than the number of sectorsare substituted in the equation of the autoregressive model, and asubsequent position error is estimated.

The above processing can be sufficiently performed with the operationspeed of the 1-chip microcomputer 17a included in the controller 17. Ifa high-speed processor is used, the operation speed is furtherincreased.

In this manner, the track-following characteristics can be preventedfrom being degraded even when the number of bytes of data per sector isincreased.

FIG. 7 shows the recording format of two adjacent sectors of a floppydisk according to another embodiment of the present invention.

In this embodiment, the operations from the readout of recordedinformation by the magnetic head to the drive control of the magnetichead by the position error signal are executed by the circuit similar tothat shown in FIG. 4, and the detailed description is omitted.

In this embodiment, a servo track (track (-1)) 32 for recording onlyservo information for positioning the magnetic head 11 is providedoutside a data track (track (1)) 30 of the floppy disk. The position ofthe servo track 32 can be arbitrarily selected and is preferable if itis provided on the outermost or innermost track of the floppy disk.

A servo information area 41 for recording the servo information isprovided at the beginning of each sector of all the data tracks. Theremaining portion 42 of the data track is defined as a data area forrecording data.

The servo pattern concerning the track 30 consists of servo information34a, recorded on the first half of the track 30 and the adjacent half ofa track (0) 31 adjacent to the outer side to the track 30, and servoinformation 34b, recorded on the second half of the track 30 and theadjacent half of a track 29 adjacent to the inner side of the track 30.The position of servo information 34b is offset from that of the servoinformation 34a. The servo information 34a and the servo information 34bhave different waveforms and can be distinguished from each other.Therefore, when the magnetic head 11 is correctly positioned at acentral portion of the track 30, an output of the servo information 34abecomes equal to that of the servo information 34b. On the other hand,when the magnetic head 11 is offset toward either the track 29 or 31,the outputs of the servo information 34a and the servo information 34bare not balanced. The track 30 can be correctly followed when theposition of the magnetic head 11 is determined to equal these outputs.

On the servo track 32, servo information 35a and servo information 35bare recorded at the positions identical to those of the servoinformation area 41 of each sector of the data track 30. Furthermore,servo information 36a and servo information 36b are recorded at anintermediate portion of the data information area 42 of the data trackin the same pattern as that of the servo information 35a and the servoinformation 35b.

The number of location of the servo information at the intermediateportion of the data area 42 is not limited to one, and the servoinformation can be recorded at two portions or more, or can becontinuous throughout the servo track.

Data is recorded in or read out from the floppy disk having the abovearrangement in the following manner.

More specifically, prior to recording or regeneration of the initialdata, the magnetic head 11 is positioned at the servo track 32 as thepreparatory operation. The servo information 35a and the servoinformation 35b are read out from the servo track 32, and informationconcerning the distortion of the floppy disk is obtained based on thereadout information 35a and the readout information 35b. When the floppydisk is deformed as shown in FIG. 1, the servo information 35a, theservo information 35b, the servo information 36a, the servo information36b, and so on of the servo track 32 are read out to obtain a directionof the distortion, i.e., whether the distortion is centripetal orcentrifugal, and the amount of distortion, i.e., the amount of positionadjustment required for the magnetic head, at the angular position ofthe rotation of the floppy disk storing these pieces of servoinformation. The obtained direction and the amount of distortion arerecorded in the memory 17B of the controller 17 as the distortioninformation.

When the above preparatory operation is completed, normal datarecording/regeneration is performed. When the amount of distortion ofthe floppy disk is small and the magnetic head 11 can read out all theservo information, the magnetic head 11 is positioned or thetrack-following operation by the magnetic head 11 is performed using aposition error signal obtained from the readout servo information. Inthe data area 42, the magnetic head 11 is maintained at the position setbased on the servo information 34a and the servo information 34b of thesector 40, and data 43 is recorded or regenerated.

When the amount of distortion of the floppy disk is large and servoinformation of a specific sector cannot be read out, the servoinformation is added using the distortion information of the servo track32 which has been stored in advance. More specifically, a direction inwhich the magnetic head 11 should move to perform the track-followingoperation is determined using the information about the distortiondirection of the servo track 32. The amount to be corrected iscalculated from the information about the distortion amount of the servotrack 32 and the ratio in diameter of the servo track 32 to target datatrack, and the magnetic head is positioned on the target track.

When one sector is long, i.e., the number of sectors is small, if themagnetic head 11 is held at a position of the sector determined by theservo information, the magnetic head 11 is deviated from the center ofthe data track in the second half of the data area, and the magnitude ofa recorded or regenerated output of the data information is decreased.In this case, the magnetic head 11 is positioned by using the distortiondirection and its amount obtained from the prestored servo information36a and the prestored servo information 36b of the intermediate portionof the sector. As a result, the data can be recorded or regeneratedwhile causing the magnetic head 11 to constantly and optimally performthe track-following operation. Even when a single track consists of asingle sector in an extreme case, the magnetic head 11 of the FDD canrecord or regenerate information while following a deformed track inaccordance with the control system of this embodiment.

When data recording/regeneration is continuously performed, theenvironmental conditions change. Therefore, it is sometimes impossibleto sufficiently perform the track-following operation by the magnetichead 11 in accordance with the distortion information of the floppy diskbased on the servo information initially read out from the servo track32. In order to cope with this, a means for updating the servoinformation of the servo track at appropriate time intervals may beused. Furthermore, every time a track-following operation for recordingor regenerating data, as described above, is performed, or after anappropriate time lapse from the track-following operation, the servoinformation 34a and the servo information 34b on the data track 30 canbe corrected, and subsequent positioning operations can be performed bythe above-described manner in accordance with the correction result. Asa result, the stable positioning/track-following operation can beperformed in accordance with the latest distortion information without aneed for a special operation for correcting the distortion informationduring a continuous recording/regenerating operation.

As described above in detail, with the track-following control system ofthe magnetic head according to the present invention, even when a floppydisk has anisotropic deformation despite that a sector servo method isadopted, the magnetic head having good track-following characteristicscan perform stable positioning. As a result, the reliability inrecording/regeneration of data information of an FDD can be increased.

What is claimed is:
 1. A track-following control system for a magneticdisk drive, comprising:a magnetic disk having a plurality of tracks eachof which is divided into a plurality of sectors and stores not less thanone track position signal indicating a position of a track in eachsector of said track; a magnetic head for writing and reading out datain and from said magnetic disk and reading out said track positionsignal from said magnetic disk; position error signal generating meansfor generating a position error signal in numbers of sectors indicatinga position error between said magnetic head and said track by using thetrack position signal read out by said magnetic head; calculating andoutputting means for calculating and outputting position error signalshaving a larger number than that of said sectors by using the positionerror signal supplied from said position error signal generating means;and positioning means for positioning said magnetic head in accordancewith said position error signals having the larger number than that ofsaid sectors, wherein said positioning means performs track-following ofsaid magnetic disk; and wherein a single track position signal isrecorded in said numbers of sectors of a single track of said magneticdisk, and said calculating and outputting means for calculating andoutputting said position error signals having the larger number thanthat of said sectors comprises generating and storing means forgenerating and storing a signal representing an estimated intra-sectorposition error by using an interpolation technique based on said errorsignals in said numbers of sectors, and wherein said calculating andoutputting means for calculating and outputting said position errorsignals of the number larger than that of said sectors comprises:obtaining means for obtaining a number of terms and correspondingcoefficients of an equation of an autoregressive model by usingsimulation and storing an obtained result, said autoregressive modelbeing used to obtain a later position error by multiplying a previous,actual position error signal and said estimated position error obtainedby interpolation by said corresponding coefficients, respectively, andadding both products; and predicting means for predicting a futureposition error by multiplying said actual position error signal and saidestimated intra-sector position error signal by said correspondingcoefficients stored, respectively, and adding both products, andoutputting a sum.